1. Field of the Invention
The present invention relates to a multiple value image input device which inputs picture images in multiple chromatic gradations. More particularly, the present invention relates to a multiple value image input device which corrects inaccuracies in the expression of chromatic grades.
2. Discussion of the Related Art
Picture image processing with a computer system is used extensively. Picture image input devices receive and transmit the existing picture images and photographs for such processing. Most known picture image input devices convert the input picture images into signals in binary values, as typically observed in the picture image input block in the conventional facsimile machine. However, such an image data processing method cannot reproduce halftones even if it can reproduce characters and line drawings. Halftone recording processes such as the "Dither Process" have been developed. Multiple value picture image input devices which are capable of performing the input of picture images in the form of data in multiple values comprise a large proportion of the Dither Process devices.
Shading correction is applied to the picture image data. Even if an identical part of an original sheet with the same degree of optical density is read, the levels of the signals output by the pixels will not be in complete agreement. Various factors, such as lack of uniformity in the sensitivity of the individual pixel detectors comprising the one-dimensional image sensor and a lack of uniformity in the quantity of light on the lines of the original sheet cause the inconsistencies.
Some image input devices are provided with a white plate (the density indicating block) arranged outside the original sheet reading area, and the one-dimensional image sensor reads the white plate before reading the original sheet. The device sets the density value so that the value read will be in agreement with the level of the white color for each of the individual pixels.
The picture images are acceptable, corrected if the correction is made with reference to the density of picture images in white color derived from the white color plate. However, multiple value image input devices process picture images on a scale of chromatic gradation composed of as many as 64 or 256 chromatic grades or tones. Differences among the individual pixels will occur in the density value corresponding to the color black if the individual pixel detectors differ in their dynamic range (namely, the difference between black and white). Even if the reference point for the white color is adjusted to the same level for the individual pixels, differences will occur. Because of this deviation, picture image data in multiple values for the individual pixels will not necessarily indicate an identical value even if the data is a gray color at an identical optical density.
This point is illustrated in FIGS. 37(a)-37(d). FIG. 37(a) illustrates a white shading correction plate 11. The white plate 11 is read with a one-dimensional image sensor 12, shown in FIG. 37 (b). The one-dimensional image sensor 12 is composed of n-pixel detectors from "1" to "n". Each image sensor 12 reads picture images pixel by pixel.
Each image pixel is expressed in a density gradation value or shading. The density gradation values are also typically referred to as chromatic gradation levels. In this example, each image pixel is ideally expressed in one of the 64 density values from "0" to "63".
FIG. 37(c) shows the dynamic ranges of the density value of four pixel detectors, the first, third, fifth, and n-th pixel detectors. In this example, the first pixel detector has a density value range from "0" to "63" from the white color plate 11 to the dark state. The first pixel is expressing the desired density values, and can accurately indicate chromatic gradation without correction.
The third pixel expresses a dynamic range from "-1" to "62". The third pixel can accurately indicate chromatic gradation with the addition of "1" to all the levels in order to adjust to "0" the density value indicated for the white color plate 11. A satisfactory correction can be made of the output from the third pixel in this example.
The fifth pixel changes its density value from "10" to "55" from the white color plate 11 to the dark state. Even if a correction is made to subtract "10" from each of the levels, the levels of the chromatic gradation indicated by the fifth pixel will be in the range from "0" to "45", as shown in FIG. 37(d). A black color is corrected to a gray color, and a gray color is corrected to a brighter gray color. Therefore, a satisfactory correction of the fifth pixel is not possible.
The n-th pixel shows a state opposite to the fifth pixel. The dynamic range of the n-th pixel is wider than the normal range extending from "-5" to "67". When a correction is made by the addition of "5" to all the density values in the range, the density values will be in the range from "0" to "72." The comparatively dark gray color will be corrected to black, while the other gray shades will be corrected to darker gray shades. Therefore, a satisfactory correction of the n-th pixel is not possible.
The description given above is based on the assumption that the sensitivity of the individual pixels show the same characteristics over the entire wavelength region. However, the sensitivity of the individual pixels may not be uniform for the individual density values. For instance, some pixel detectors have a higher sensitivity in the lower brightness region of the scale of chromatic gradation, while other pixel detectors have higher sensitivity in the higher brightness region of the scale. The gray color which should be a constant density value may be rendered in gray shades with different density values because the individual pixel detectors have different responses to the changes in brightness even though they have identical dynamic range.
Corrections made to compensate for the differences in the sensitivity of the individual pixel detectors in the one-dimensional image sensor have been discussed. The problem exists in which corrections made for the influences of other factors such as the quantity of light.
An example of the actual output levels from a multiple value image input device is shown by the solid line 14 in FIG. 38. When the density of the picture image data is represented in the form of data in eight bits (in 256 stages), the actual output level is expressed in six bits (in 64 stages). The one-dot chain line 15 indicates the desired output characteristics. Thus, the output characteristics of a multiple value image input device are not simple. A variety of factors are intertwined in output characteristics formation, and proper correction of chromatic gradation for the reproduction of halftone is, therefore, difficult.
Fluctuations also occur while reading each line in the subsidiary direction of scanning with the one-dimensional image sensor. The following factors can be pointed out as the causes of such fluctuations changes in the distance from the original sheet due to vibrations in the scanner which moves the one-dimensional image sensor in the subsidiary scanning direction; changes in the light quantity accumulating time due to fluctuations in the speed of the scanner caused by vibrations in the unit; and changes in the quantity of light from the light source during the subsidiary scanning operation.